265 research outputs found
Ferromagnetism in Oriented Graphite Samples
We have studied the magnetization of various, well characterized samples of
highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite
to investigate the recently reported ferromagnetic-like signal and its possible
relation to ferromagnetic impurities. The magnetization results obtained for
HOPG samples for applied fields parallel to the graphene layers - to minimize
the diamagnetic background - show no correlation with the magnetic impurity
concentration. Our overall results suggest an intrinsic origin for the
ferromagnetism found in graphite. We discuss possible origins of the
ferromagnetic signal.Comment: 11 figure
Mapping Neural Circuits with Activity-Dependent Nuclear Import of a Transcription Factor
Nuclear factor of activated T cells (NFAT) is a calcium-responsive transcription factor. We describe here an NFAT-based neural tracing method—CaLexA (calcium-dependent nuclear import of Lex A)—for labeling active neurons in behaving animals. In this system, sustained neural activity induces nuclear import of the chimeric transcription factor LexA-VP16-NFAT, which in turn drives green fluorescent protein (GFP) reporter expression only in active neurons. We tested this system in Drosophila and found that volatile sex pheromones excite specific neurons in the olfactory circuit. Furthermore, complex courtship behavior associated with multi-modal sensory inputs activated neurons in the ventral nerve cord. This method harnessing the mechanism of activity-dependent nuclear import of a transcription factor can be used to identify active neurons in specific neuronal population in behaving animals
Amino Acid Residues Contributing to Function of the Heteromeric Insect Olfactory Receptor Complex
Olfactory receptors (Ors) convert chemical signals—the binding of odors and pheromones—to electrical signals through the depolarization of olfactory sensory neurons. Vertebrates Ors are G-protein-coupled receptors, stimulated by odors to produce intracellular second messengers that gate ion channels. Insect Ors are a heteromultimeric complex of unknown stoichiometry of two seven transmembrane domain proteins with no sequence similarity to and the opposite membrane topology of G-protein-coupled receptors. The functional insect Or comprises an odor- or pheromone-specific Or subunit and the Orco co-receptor, which is highly conserved in all insect species. The insect Or-Orco complex has been proposed to function as a novel type of ligand-gated nonselective cation channel possibly modulated by G-proteins. However, the Or-Orco proteins lack homology to any known family of ion channel and lack known functional domains. Therefore, the mechanisms by which odors activate the Or-Orco complex and how ions permeate this complex remain unknown. To begin to address the relationship between Or-Orco structure and function, we performed site-directed mutagenesis of all 83 conserved Glu, Asp, or Tyr residues in the silkmoth BmOr-1-Orco pheromone receptor complex and measured functional properties of mutant channels expressed in Xenopus oocytes. 13 of 83 mutations in BmOr-1 and BmOrco altered the reversal potential and rectification index of the BmOr-1-Orco complex. Three of the 13 amino acids (D299 and E356 in BmOr-1 and Y464 in BmOrco) altered both current-voltage relationships and K+ selectivity. We introduced the homologous Orco Y464 residue into Drosophila Orco in vivo, and observed variable effects on spontaneous and evoked action potentials in olfactory neurons that depended on the particular Or-Orco complex examined. Our results provide evidence that a subset of conserved Glu, Asp and Tyr residues in both subunits are essential for channel activity of the heteromeric insect Or-Orco complex
Mechanism of grain-boundary magnetoresistance in Fe
The magnetotransport properties of magnetite films with
different microstructures were investigated in order to identify
prerequisites for the attainment of a large tunnelling
magnetoresistance in polycrystalline samples. Epitaxial films on
MgAl2O4, polycrystalline films on Al2O3 and rough
MgAl2O4 substrates and a polycrystalline
La0.7Ca0.3MnO3 film on MgO were compared. Although
grain boundaries induce a large high-field magnetoresistance in
magnetite films, the low-field magnetoresistance characteristic for
spin-polarized tunnelling was virtually absent in these samples. Two
factors might be responsible for this behaviour: (1) grain
boundaries in magnetite are conducting and do not form tunnelling
barriers and (2) the spin-polarization near grain boundaries is
suppressed due to non-stoichiometry
Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures
We have measured the low-temperature (K) specific heat and
heat release of small amorphous and crystalline SiO2 particles
embedded in Teflon and of Vycor. The temperature and time dependence
of these properties have been interpreted in terms of the tunneling
model. We found that the particle size influences the density of
states of tunneling systems of the composite. The smaller the size
of the particles the larger is the density of states of tunneling
systems P0. Quartz grains with dimensions in the micrometer
range show similar glass-like properties as vitreous silica. In
comparison with bulk vitreous silica, Vycor shows a much larger
P0 in agreement with the behavior we found for small SiO2
particles. We discuss the implication of our results on the origin of
the universal low-temperature properties of glasses
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